1. Field of the Invention
The present invention generally relates to a polarized wave scrambler and an optical signal transmission apparatus in which a wave transmission of an optical communication system does not have polarization dependent characteristics, and more particularly to a polarized wave scrambler and an optical signal transmission apparatus in which polarization mode dispersion that degrades a transmitted waveform is suppressed, and which can be easily manufactured and are low-cost and stable.
In optical communication systems, first put to practical use around 1950 in long distance optical communication systems, a regenerative repeater that has so called 3R functions, namely reshaping, retiming and regenerating, is used to relay a transmission signal.
In this system, the structure of the regenerative repeater is complex and therefore, the regenerative repeater has many adjustment points to implement the 3R functions in the regenerative repeater. Therefore, it is hard to reduce the cost of the regenerative repeater. Further, in the regenerative repeater, an optical signal, which is not usually affected by an electromagnetic wave, is converted to an electrical signal. Then, the reshaping of the electrical signal is performed and the reshaped electrical signal is re-converted to an optical signal to relay the signal. Therefore, it is required to carefully design the regenerative repeater with respect to electrical characteristics and arrangement of elements in order to avoid an increase of a code error rate due to the electro-magnetic interference, and it is also hard to reduce the cost of the regenerative repeater.
In the 1980s, an optical fiber amplifier using an erbium doped optical fiber, in which ions of a rare earth element, especially the ions of erbium (Er) are doped, was developed. The optical fiber amplifier has been used in place of the regenerative repeater, and also is used as an output amplifier for terminal stations or as an amplifier provided just before a branch point to a plurality of optical fibers.
It is possible to reduce the number of parts and the number of the adjustment points in the repeater by means of employing the optical fiber amplifier using the erbium doped optical fiber as the repeater in the optical communication system. Further, because it is not required to carefully design the repeater with respect to electrical characteristics and arrangement of elements, it is possible to reduce the cost of the repeater and to raise the reliability.
The optical fiber used as a trunk transmission line and the optical fiber amplifier using the erbium doped optical fiber has polarization dependent loss (PDL) and polarization mode dispersion (PMD) which basically correspond to group delay time if the electrical signal is transmitted. The PDL and the PMD degrade an envelope waveform of the optical signal that is modulated by the electrical signal. The envelope waveform of the optical signal, which is modulated by the electrical signal, is reproduced at the receiving side. Therefore, transmission performance is degraded according to the PDL and the PMD.
Especially, a high speed optical communication system is needed because communication traffic becomes great. In the high speed optical communication system, duration interval of one optical pulse is short and the effect of the PDL and the PMD cannot be negligible. Therefore, the polarized wave scrambler is employed in the optical communication system to propagate the light in the optical fiber without maintaining a constant polarization. The polarized wave scrambler plays an important roll as mentioned above, but it is also required to be manufactured with low-cost and to be stable, and further, it is also required not to degrade the transmission characteristics.
The present invention provides a polarized wave scrambler in which the polarization mode dispersion, which degrades a transmitted waveform, is suppressed, and which can be easily manufactured, and an optical signal transmission apparatus in which the polarization mode dispersion is suppressed using such polarized wave scrambler.
2. Description of the Related Art
Conventionally, technologies to polarized-wave-scramble a light by means of controlling polarization are as follows,
(1) a polarized wave scrambler in which optical polarization elements are mechanically moved to control the polarization,
(2) a polarized wave scrambler in which the polarization angle of the linearly polarized optical signal is set between a high speed axis and a low speed axis of a polarization maintaining optical fiber and then, the optical signal is supplied to the polarization maintaining optical fiber, and
(3) a polarized wave scrambler in which a linearly polarized optical signal is supplied to a Lithium Niobate (LiNbO3) modulator, and a phase of the optical signal propagated through one waveguide is controlled using electro-optic effect, by means of which the phase of the light is controlled by voltage, or thermo-optic effect, by means of which the phase of the light is controlled by heat.
The polarized wave scrambler (1) has a complex structure and therefore it is hard to reduce cost. Further, mechanically moving parts may cause a problem of durability for the polarized wave scrambler and therefore, the reliability is low. For the polarized wave scrambler (3) using the Lithium Niobate (LiNbO3) modulator, the cost of the Lithium Niobate (LiNbO3) modulator is high.
Therefore, the polarized wave scramblers (1) and (3) are not suitable to manufacture a low-cost polarized wave scrambler. However, the polarized wave scrambler (2) is suitable to manufacture a low-cost polarized wave scrambler.
First, an example of the polarization maintaining optical fiber and the polarization mode dispersion will be explained.
FIG. 1 shows an example of the polarization maintaining optical fiber and a sectional diagram of the polarization maintaining optical fiber. This polarization maintaining optical fiber is called “a panda-fiber” and which panda-fiber was developed in Japan.
In FIG. 1, reference numeral 100 shows the polarization maintaining optical fiber, reference numeral 101 shows its core, reference numeral 102 shows its cladding, reference numeral 103 and 103a show stress generating parts that are formed at both side of the core 101 in the cladding 102.
The stress generating parts 103 and 103a consist of B2O3 doped silica glass. The coefficient of thermal expansion of the B2O3, doped silica glass is several times greater than that of silica glass. After optical fiber drawing is performed, as the optical fiber shrinks when the temperature of the optical fiber decreases, the stress stretches the core in the directions of a line connecting between and toward the stress generating parts 103 and 103a. Therefore, compressive stress compresses the core in the direction perpendicular to the line connecting between the stress generating parts 103 and 103a. As a result, a birefringent optical fiber, which has different refractive indexes for the propagating light in the core depending on the direction, can be produced.
The refractive index in the direction of the line connecting between the stress generating parts 103 and 103a is low and the refractive index in the direction perpendicular to the line connecting between the stress generating parts 103 and 103a is high. Therefore, the propagation velocity of the light in the direction of the line connecting between the stress generating parts 103 and 103a is high and the propagation velocity of the light in the direction perpendicular to the line connecting between the stress generating parts 103 and 103a is low. An axis in the direction of the line connecting between the stress generating parts 103 and 103a is called the high-speed axis and another axis in the direction perpendicular to the line connecting between the stress generating parts 103 and 103a is called the low-speed axis.
When a plane of polarization of the linearly polarized optical signal is set at an angle to either the high-speed axis or the low-speed axis of the polarization maintaining optical fiber and then the linearly polarized optical signal is supplied to the polarization maintaining optical fiber, the plane of the polarization of the linearly polarized optical signal which propagates through the polarization maintaining optical fiber is kept constant because rotation of the plane of the polarization does not occur. As a result, the polarization state is maintained. Therefore, this optical fiber is called a polarization maintaining optical fiber.
When the plane of polarization of the linearly polarized optical signal is set to an angle between the high-speed axis and the low-speed axis of the polarization maintaining optical fiber, for example 45 degrees, and then the linearly polarized optical signal is supplied to the polarization maintaining optical fiber, the linearly polarized optical signal is divided into the high-speed axis component and the low-speed axis component and each component propagates through the optical fiber. Because the propagation velocity of the light in the direction of the high-speed axis is different from the propagation velocity of the light in the direction of the low-speed axis, it is possible to produce any polarization state at the output of the polarization maintaining optical fiber according to the length of the polarization maintaining optical fiber. However, when the polarization maintaining optical fiber is simply used, the optical output waveform is degraded. This degradation of waveform is caused by the polarization mode dispersion (PMD).
FIG. 2 shows the polarization mode dispersion.
As shown in FIG. 2, when the plane of polarization of the linearly polarized optical signal is set to an angle between the high-speed axis and the low-speed axis of the polarization maintaining optical fiber, for example 45 degrees, and then the linearly polarized optical signal is supplied to the polarization maintaining optical fiber. Because the propagation velocity of the light in the direction of the high-speed axis is different from the propagation velocity of the light in the direction of the low-speed axis, the constant phase difference between the high-speed axis component and the low-speed axis component is accumulated. This accumulated phase difference is the polarization mode dispersion.
A normal optical fiber has a wavelength dispersion characteristic, so that the propagation velocity of light is different according to the wavelength of the light. Usually, full width at half level of the propagated light pulse through the optical fiber becomes broader than that of the input light pulse, and the rising-time and the falling-time of the pulse waveform are increased.
FIG. 2 shows a waveform degraded by both the polarization mode dispersion (PMD) and the wavelength dispersion. One peak of the light pulse is divided into two peaks because of the speed difference caused by the polarization mode dispersion (PMD) and the light pulse is broadened because of the wavelength dispersion, and then the light pulse having such waveform is supplied from the polarization maintaining optical fiber.
FIG. 3 shows a conventional block diagram to polarized-wave-scramble the light according to the prior art, and also shows a light source and a driving circuit, and so on.
In FIG. 3, reference numeral 1 shows the light source that supplies the linearly polarized optical signal, for example a laser diode. Reference numeral 1—1 shows a pig-tail fiber that guides the light supplied from the light source 1 outside the light source 1.
Reference numeral 2 shows a light source driving circuit that supplies a driving current to the light source 1 and the light source driving circuit 2 is usually composed of a circuit of a current-switch-type.
Reference numeral 3 shows an oscillator that supplies a sinusoidal wave having a frequency of f1, which sinusoidal wave modulates the driving current supplied from the light source driving circuit 2.
Reference numeral 4 shows a polarization maintaining optical fiber that is spliced to the pig-tail fiber 1—1. At the splicing point between the polarization maintaining optical fiber 4 and the pig-tail fiber 1—1, the polarization maintaining optical fiber 4 and the pig-tail fiber 1—1 are spliced such that the plane of polarization of the linearly polarized optical signal supplied from the light source 1 is set to an angle between the high-speed axis and the low-speed axis of the polarization maintaining optical fiber 4, for example 45 degrees. The polarized-wave-scrambler is constructed by the polarization maintaining optical fiber 4.
As shown in FIG. 3, features of this construction are that first, the light source 1 is driven by the output current supplied from the light source driving circuit 2 modulated by the sinusoidal signal having a frequency of f1, supplied from the oscillator 3, and second, at the splicing point between the polarization maintaining optical fiber 4 and the pig-tail fiber 1—1, the polarization maintaining optical fiber 4 and the pig-tail fiber 1—1 are spliced such that the plane of polarization of the linearly polarized optical signal supplied from the light source 1 is set to an angle between the high-speed axis and the low-speed axis of the polarization maintaining optical fiber 4, for example 45 degrees.
FIG. 4A through FIG. 4D show the principle of the polarized wave scramble shown in FIG. 3, and FIG. 5 shows phase difference between the high-speed axis component and the low-speed axis component of the light propagating in the optical fiber generated by the polarization maintaining optical fiber as shown in FIG. 3.
FIG. 4A shows the high-speed axis and the low-speed axis of the polarization maintaining optical fiber. The x-axis denotes the high-speed axis and the y-axis denotes the low-speed axis.
FIG. 4B shows an electrical field Ex in the x-axis direction and an electrical field Ey in the y-axis direction of the input light at the input point of the polarization maintaining optical fiber 4. As described above, the plane of polarization of the input light is set to the angle between the high-speed axis and the low-speed axis of the polarization maintaining optical fiber 4, the electrical field Ex in the high-speed axis direction and the electrical field Ey in the low-speed axis direction of input light are applied at the input point of the polarization maintaining optical fiber. Especially, the electrical filed Ey in the low-speed (y-axis) direction is equal to the electrical filed Ex in the high-speed (x-axis) direction if the plane of polarization of the input light is set to the angle which is equal to 45 degrees for both the high-speed axis and the low-speed axis.
FIG. 4C shows the electrical field Ex in the x-axis direction and the electrical field Ey in the y-axis direction of the output light at the output point of the polarization maintaining optical fiber 4. As described above, a phase shift value for the high-speed axis is different from that of the low-speed axis. The phase shift value for the high-speed axis is δx and of the phase shift value of the low-speed axis is δy. As a result, the electrical field Ex in the x-axis direction and the electrical field Ey in the y-axis direction of the output light at the output point of the polarization maintaining optical fiber 4 are shown in FIG. 4C.
FIG. 4D shows a phase difference between the phase shift value of the high-speed axis direction and the phase shift value of the low speed axis direction. As described above, the phase difference is a trigonometric function of the frequency f1, because the output light from the light source is modulated by the sinusoidal signal having the frequency of f1.
FIG. 5A shows a graph of the phase difference generated by the polarization maintaining optical fiber 4. FIG. 5B shows the phase difference value which is approximated to a phase difference component φ(f0) at the frequency f0 and a value which is a differential coefficient of the phase φ at the frequency f0 multiplied by the modulation frequency f1. In this specification, the phase difference component φ(f0) at the frequency f0 is called “a constant component” and the value which is a differential coefficient of the phase φ at the frequency f0, multiplied by the modulation frequency f1 is called “a modulated component” or “a frequency characteristic of the phase difference”. The polarized-wave-scramble can be performed by the modulated component in the polarization maintaining optical fiber.
The constant component of the phase difference generated in the polarization maintaining optical fiber corresponds to the constant difference between the propagation velocity of the light in the high-speed axis direction and the propagation velocity of the light in the low-speed axis direction, and therefore, also corresponds to the polarization mode dispersion (PMD). There is a problem in which it is possible to polarized-wave-scramble the input light by the polarization maintaining optical fiber, but it is not possible to prevent the degradation of the waveforms caused by the polarization mode dispersion (PMD).